Peripheral devices such as disk drives used in processor-based systems may be slower than other circuitry in those systems. There have been many attempts to increase the performance of disk drives. However, because disk drives are electromechanical, there may be a finite limit beyond which performance cannot be increased. One way to reduce the information bottleneck at the peripheral device, such as a disk drive, is to use a cache. A cache is a memory device that logically resides between a device, such as a disk drive, and the remainder of the processor-based system. A cache is a memory device that serves as a temporary storage area for the device, such as the disk drive. Frequently accessed data resides in the cache after initial access. Subsequent accesses to the same data may be made to the cache instead of to the disk drive.
Generally, two types of disk cache are used, write-through cache and write-back cache. Write-through disk cache means that the information is written both to the cache and to the corresponding disk drive. Write-back disk cache means that information is only written to the cache, and the corresponding disk drive is subsequently updated when the corresponding cache line is flushed. Write-back cache is faster than write-through cache but may cause coherency problems since the data in the cache may be different than in the corresponding disk drive until the corresponding cache line is flushed. A cache line of data is dirty if the data in the cache line has been updated by the system but the corresponding disk drive has not been updated. A clean cache line is a line of data in a cache that has been flushed (updated) to the corresponding disk drive.
In a system which includes non-volatile write-back disk cache, dirty cache lines may be flushed as part of a system shutdown procedure so that the cache data is coherent with the disk drive data. Coherency at shutdown protects against a cache removal or cache failure while the system is turned off. However, flushing large cache during shutdown may require extensive writing to random locations on the disk drive which may require a lot of time, perhaps minutes, to execute.
Thus, a need exists for alternative ways of flushing a cache during a system shutdown.